1. Field of the Invention
The present invention relates to a circuit for controlling RF PAs (Radio Frequency Power Amplifiers), and more specifically, to an RF PA controller circuit that controls the supply voltage of a PA using a closed amplitude control loop with an amplitude correction signal.
2. Description of the Related Art
RF transmitters and RF power amplifiers are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF power amplifiers are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life on these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA circuits, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time.
FIG. 1 illustrates a conventional RF transmitter circuit, including a transmitter integrated circuit (TXIC) 102 and an external power amplifier (PA) 104. For example, the RF transmitter circuit may be included in a cellular telephone device using one or more cellular telephone standards (modulation techniques) such as UMTS (Universal Mobile Telephony System) or CDMA (Code Division Multiple Access), although the RF transmitter circuit may be included in any other type of RF electronic device. For purposes of illustration only, the RF transmitter circuit will be described herein as a part of a cellular telephone device. The TXIC 102 generates the RF signal 106 to be amplified by the PA 104 and transmitted 110 remotely by an antenna (not shown). For example, the RF signal 106 may be an RF signal modulated by the TXIC 102 according to the UMTS or CDMA standard.
The RF power amplifier 104 in general includes an output transistor (not shown) for its last amplification stage. When an RF modulated signal 106 is amplified by the RF PA 104, the output transistor tends to distort the RF modulated signal 106, resulting in a wider spectral occupancy at the output signal 110 than at the input signal 106. Since the RF spectrum is shared amongst users of the cellular telephone, a wide spectral occupancy is undesirable. Therefore, cellular telephone standards typically regulate the amount of acceptable distortion, thereby requiring that the output transistor fulfill high linearity requirements. In this regard, when the RF input signal 106 is amplitude-modulated, the output transistor of the PA 104 needs to be biased in such a way that it remains linear at the peak power transmitted. This typically results in power being wasted during the off-peak of the amplitude of the RF input signal 106, as the biasing remains fixed for the acceptable distortion at the peak power level.
Certain RF modulation techniques have evolved to require even more spectral efficiency, and thereby forcing the RF PA 104 to sacrifice more efficiency. For instance, while the efficiency at peak power of an output transistor of the PA 104 can be above 60%, when a modulation format such as WCDMA is used, with certain types of coding, the efficiency of the RF PA 104 falls to below 30%. This change in performance is due to the fact that the RF transistor(s) in the RF PA 104 is maintained at an almost fixed bias during the off-peak of the amplitude of the RF input signal 106.
Certain conventional techniques exist to provide efficiency gains in the RF PA 104. One conventional technique is EER (Envelope Elimination and Restoration). The EER technique applies the amplitude signal (not shown in FIG. 1) and the phase signal (not shown in FIG. 1) of the RF input signal 106 separately to 2 ports of the power amplifier 104, i.e., its supply voltage port (Vcc) 108 and its RF input port 107, respectively. However, the EER technique fails to provide significant efficiency gains, because the supply voltage 108 cannot be varied in an energy-efficient way to accommodate the large variations in the amplitude signal of the RF input signal 106 and thus it fails to provide a substantial energy efficiency gain while maintaining the required linear amplification of the RF signal in the RF PA 104. This is mainly due to the difficulty in realizing a fast, accurate, wide range, and energy efficient voltage converter to drive the supply voltage of the RF PA 104.
The conventional EER technique can function better only if a variable power supply with a very large variation range is used to adjust the supply voltage based on the amplitude signal of the RF input signal 106, while not reducing the efficiency of the RF transmitter by power consumed by the power supply itself. However, the variable power supply, which is typically comprised of a linear regulator (not shown in FIG. 1) that varies its output voltage on a fixed current load such as the PA in linear mode, by principle reduces the supply voltage at constant current and by itself consumes the power resulting from its current multiplied by the voltage drop across the linear regulator when there is a large drop in the amplitude signal of the RF input signal 106. This results in no change in the overall battery power being consumed by the RF transmitter, because any efficiency gained in the RF PA 104 is mostly lost in the linear regulator itself. Variations of the EER technique, such as Envelope Following and other various types of polar modulation methods, likewise fails to result in any significant gain in efficiency in the RF transmitter, because the supply voltage is likewise adjusted based on the amplitude signal of the RF input signal 106 which inherently has large variations and thus has the same deficiencies as described above with respect to conventional EER techniques.
Quite often, the conventional methods of controlling a PA fail to address the amplitude-to-phase re-modulation (AM-to-PM) which occurs in a non-frequency linear device such as a PA. Thus, the conventional methods are not suitable for the common types of PAs for use in common mobile telephony or mobile data systems because the required spectral occupancy performance is compromised by the AM to PM distortion.
PAs are typically used in conjunction with band pass filters that have a high electric coefficient of quality. These filters are typically of the SAW (surface acoustic wave) type. Due to their high coefficient of quality, the filters exhibit a relatively high group delay. The group delay makes it very difficult for a correction loop to work around the arrangement of the SAW filter and the PA while still meeting the high bandwidth requirements needed for the correction of the AM-to-PM.
In addition, it is advantageous for a RF PA circuit to detect and act upon load variations at the antenna to which the RF PA circuit is coupled. Especially, it is advantageous if the RF PA circuit acts upon the load variation in a way which does not require an adjustment of power from the TXIC 102. Load variations can occur, for example, when the antenna is placed near a metal object and the normal electromagnetic field pattern of the antenna is disturbed. Such load variations would typically cause reduction in the radiated power from the antenna, in part due to the absorption of the antenna's radiated energy by the object in its proximity, and also in part due to the resulting difference between the expected and actual load impedance driven by the PA (which is referred to herein as “antenna impedance mismatch”).
In some radio systems, the receiving base station can detect the level of radiated power received from the transmitting radio and command the radio to increase the power level from its RF PA to compensate for load variations at the antenna of the transmitting radio. However, this requires intervention from and communication with the receiving base station, and the transmitting radio itself is not able to adjust the power level on its own. In addition, the RF PA may already be transmitting at its maximum expected power level and therefore not be able to honor the command from the receiving base station to increase the transmitting power level. The RF PA cannot increase its output power beyond its maximum expected power level, because doing so would increase distortion in the signal amplified by the RF PA to unacceptable levels.
Of course, the RF PA may be designed for a higher expected maximum power level and therefore generate a higher output power level to compensate for load variations at the antenna without increasing distortion to unacceptable levels. However, such an RF PA would suffer from poorer efficiency when operated at normal power levels (when the antenna is not subject to load variation), because the RF PA in such case would have to operate at a greater backoff from its peak output power and thus operate at a greater distance from saturation. Thus, a tradeoff should be made between the efficiency of the RF PA under normal operating power levels and the ability of the RF PA to supply additional power to compensate for load variations at the antenna.
The RF output power leveling circuit may erroneously reduce the power of the RF PA when there are load variations at the antenna because of the directional coupler that may be employed in the RF PA circuit in line with the output of the RF PA. The RF output power leveling circuit is commonly employed in cellular radios and typically employs a directional coupler to measure, regulate, or control the output power from the RF PA. Typically, the output power from the forward coupled port of the directional coupler is correlated tightly to the radiated power from the antenna. However, when there are load variations at the antenna, the directional coupler may report a power level higher than the actual radiated power at the antenna, because the directional coupler does not measure the reduction in power delivered by the RF PA caused by the antenna impedance mismatch. Thus, the RF output power leveling circuit may erroneously reduce the power of the RF PA when there are load variations at the antenna. In addition, antenna impedance mismatch seen at the RF PA output may cause an increase in power dissipated by the PA, resulting in undesirable heating in the RF PA circuit.
Thus, there is a need for an RF PA system that is efficient over a wide variety of modulation techniques and results in a significant net decrease in power consumption by the RF PA circuit. There is also a need for a PA controller that can correct the AM to PM effects, while not relying on a PA specially designed for low AM to PM at the expense of efficiency. In addition, there is a need for a PA controller that can exclude the use of SAW filters from the path of the correction loop in the PA circuitry. There is also a need for an RF PA system that can be designed to operate at a higher maximum expected output power level to compensate for load variations at the antenna, without reduced efficiency operating at normal power levels. There is also a need for an RF PA system that can compensate for antenna load variation and an RF PA system that is protected against excessive power dissipation that can be caused by antenna impedance mismatch. Finally, there is a need for an RF PA system that can minimize the distortion in the PA output when an output match compensation circuit is employed.